The technical field of the present invention is the formation of articles from sheet materials, and more particularly the formation of multi-compartmented trays from paperboard stock.
The formation of trays having several compartments separated by ribs whose height approaches that of the tray sidewall is an especially difficult problem. The stresses created in the rib regions during forming of the tray often cause the material to tear, tending to make the production of such trays unreliable and uneconomical. One early approach to the solution of the problem is disclosed in U.S. Pat. No. 2,760,231, issued to St. Clair. That patent discloses forming the tray in two stages, first forming the central portion of the tray, where the ribs are located, and then forming the sidewall.
Another approach is disclosed in U.S. Pat. Nos. 4,149,841 and 4,246,223, issued to Patterson. Patterson proposed preforming the central portion of each compartment of the tray and then completing the forming of the tray in a second step.
Despite the cited patents, however, no way has yet been disclosed to form deep trays, particularly those having ribs substantially as high as the sidewall, with an acceptably small rate of tearing and other defects.
In addition to tearing in the region of the ribs, known methods are prone to bowing (curvature of the tray bottom, especially in the vicinity of the rib separating the two smaller compartments in three-compartment trays), and to the presence of aesthetically unacceptable parting lines that occur when a compartment is formed in two stages.
It would be highly desirable to have a method of forming trays from paperboard and similar stock without the common problems of tearing, bowing and parting lines.
The principal object of the invention is therefore to provide an apparatus and a method for forming compartmented trays from sheet material, especially paperboard stock, free from the shortcomings of the prior art and capable of producing a high-quality product with a high degree of reliability and little wastage.
Another object of the invention is to provide such a method and apparatus capable of being used to form such trays having ribs substantially as high as the tray sidewalls.